recovering space on my phone

 I've had this Oppo phone for some time now (like 2016) and its been a trooper, I purchased it with a smaller amount of storage memory 16GB because it had an SD card slot which I could put in anything I liked (and currently have 128GB in there) and could put my bulkier media, douments and camera images onto that (which I also think is a better safety strategy).

Recently however its been giving me messages that it was running low on storage and I tried various things like:

•  moving files (there weren't many) over to the SD card
• clearing cache on all apps (wasn't really much in there)
• removing a few bulkier apps that I don't use much (Uber, AirBnB ...)

but to no avail, it was always nearly full and I at most gained a GB

It transpired that I needed to do a transfer over to my "spare" phone (identical model and spec) which I'd bought used in excellent condition for $50 a year or so back (for Justin Case), so I did a backup of all settings, apps and data onto the SD, transferred it and then restored.

My apps were all there BUT one important thing became clear, I had a shit ton more space

like what the actual ... ?

My only rational explanation for this is that there must be a shit ton of crap thats up in "protected" areas of the phone (this phone is NOT rooted) that somehow were taking up space. Candidates for this could include many things but for instance:

• every time an app is updated the downoladed APK file may be kept somewhere and never deleted
• each OS update may do the same
• apps may be stored in "deltas" allowing roll back  but the developers were too lazy to manage that (so they're all there)
• log files (as I've found a few ones such as the full log of every OTG connection (date, data transferred and whatnot) I've ever done

Previous explorations of the state of storage in SD 0 has always looked pretty much like this:

with that data (this is a current view) nowhere near being sufficiient to explain the contents of the entire system being what it is, the visible material adds up to under 400MB, yet its reporting that 4.14GB is used (which must be all the installed apps).

So...

if you have a phone with a compact amount of storage then maybe a backup, full factory reset and restore will help you out too.

Hope that Helps